The present invention relates to terbium- or lutetium-containing phosphors and scintillators having a garnet structure activated with rare-earth metal ions useful for the detection of high-energy radiation. In particular, the present invention relates to a terbium or lutetium aluminum oxide garnet X-ray phosphor or scintillator activated with cerium. The present invention also relates to X-ray detectors and detection systems incorporating an X-ray phosphor or scintillator comprising a terbium- or lutetium-containing garnet activated with rare-earth metal ions.
The terms xe2x80x9cphosphorxe2x80x9d and xe2x80x9cscintillatorxe2x80x9d are used herein in an interchangeable way to mean a solid-state luminescent material that emits visible light in response to stimulation by high-energy radiation such as X, xcex2, or xcex3 radiation. The term xe2x80x9chigh-energy radiationxe2x80x9d means electromagnetic radiation having energy higher than that of ultraviolet radiation. Solid-state scintillator materials are in common use as component of radiation detectors in apparatuses such as counters, image intensifiers, and computed tomography (xe2x80x9cCTxe2x80x9d) scanners. Scintillator materials especially find widespread use in X-ray detectors. One embodiment of the present generation of solid-state ceramic scintillators comprises oxide mixtures in which a rare-earth oxide is present as an activator, along with various combined matrix elements, which are also usually rare-earth oxides. Other metallic compounds may also be present as additives for specific purposes. These scintillators have been characterized by the advantageous properties of high efficiency, moderate decay time, low afterglow and little or no radiation damage upon exposure to high X-ray doses.
One important property of CT systems is scan time which is the time required for a CT system to scan and acquire an image of a slice of the object under observation. Scan times of CT systems are related to primary decay time (sometimes simply xe2x80x9cdecay timexe2x80x9dhereinafter) of the scintillator roughly by a factor of 1000. Thus, a scintillator having a decay time of 1 millisecond will typically produce a scan time of about 1 second. The scanning units containing the present generation of scintillators have scan times on the order of 1 second, and in any event no lower than about 0.7 second.
In future generations of CT scanners and the like, shorter scan times are desired. This is true because decreasing scan time makes possible an increase in patient volume covered in a given time or an increase in the number of scans within a single breath hold. Also, it reduces image blurring due to motion of internal organs and of non-cooperating patients, including pediatric patients.
Shorter scan times are achievable if the primary decay time of the phosphor or scintillator is shortened. In general, scan time in seconds is associated with a primary decay time of an equal number of milliseconds. As the speed of data processing in CT scanners increases due to advances in electronic circuit designs, it is desired to have faster scintillators, i.e., shorter time between receipts of stimulating radiation pulses so to fully take advantage of the capability of the scanner. Therefore, any measurable percentage decrease in decay time from that exhibited by the present generation of ceramic scintillators would be a distinct improvement, particularly when accompanied by the other advantageous properties described above.
Among the preferred scintillator compositions in the present generation of CT scanners are the ceramic scintillators employing at least one of the oxides of lutetium, yttrium, and gadolinium as matrix materials. These are described in detail, for example, in U.S. Pat. Nos. 4,421,671; 4,473,513; 4,525,628; and 4,783,596. They typically comprise a major proportion of yttria (Y2O3), up to about 50 mole percent gadolinia (Gd2O3) and a minor activating proportion (typically about 0.02-12, preferably about 1-6 and most preferably about 3 mole percent) of a rare earth activator oxide. Suitable activator oxides, as described in the aforementioned patents, include the oxides of europium, neodymium, ytterbium, dysprosium, terbium, and praseodymium. Europium-activated scintillators are often preferred in commercial X-ray detectors by reason of their high luminescent efficiency, low afterglow level, and other favorable characteristics. Europium is typically present therein in amounts up to 30 and most often up to about 12, preferably in the range of 1-6 and most preferably about 3 mole percent. Decay times of such scintillators are on the order of 0.9-1.0 millisecond. However, such decay times still leave much to be desired.
The search thus continues for ceramic scintillator compositions having shorter decay times in combination with the aforementioned other advantageous properties.
The present invention provides improved scintillator compositions comprising a terbium- or lutetium-containing garnet activated with at least one rare-earth metal. The scintillator compositions are useful in the detection of high-energy radiation, such as X, xcex2, or xcex3 radiation. Particularly, the scintillators of the present invention have higher light output, reduced afterglow, short decay time, and high X-ray stopping power in X-ray detection applications.
According to one aspect of the present invention, the scintillator compositions comprise terbium-containing garnet activated with at least one rare-earth metal having a general formula of (G1-x-yAxREy)aDzO12, wherein G is at least one metal selected from the group consisting of Tb and Lu; A is a member selected from the group consisting of Y, La, Gd, Lu, and Yb when G is Tb, and selected from the group consisting of Y, La, Gd, Tb, and Yb when G is Lu; RE is at least one member selected from the group consisting of Ce, Pr, Nd, Sm, Eu, Dy, Ho, Er, and Tm; D is at least one member selected from the group consisting of Al, Ga, and In; a is in the range from about 2.8 to and including 3; x is in the range from 0 to about 0.5; y is in the range from about 0.0005 to about 0.2; and z is in the range from about 4 to and including 5. In one aspect of the present invention 4 less than z less than 5.
According to another aspect of the present invention, a method for producing a rare earth-activated garnet scintillator containing Tb or Lu useful for a detection of X, xcex2, or xcex3 radiation comprises the steps of: (1) providing amounts of oxygen-containing compounds of at least one first metal selected from the group consisting of terbium and lutetium; oxygen-containing compounds of at least one rare-earth metal selected from the group consisting of Ce, Pr, Nd, Sm, Eu, Dy, Ho, Er, and Tm; and oxygen-containing compounds of at least one second metal selected from the group consisting of Al, Ga, and In; the amounts of oxygen-containing compounds being selected such that the final composition of the scintillator is achieved; (2) mixing together the oxygen-containing compounds to form a mixture; (3) optionally adding at least one fluxing compound selected from the group consisting of halides and carbonates of Tb, Al, Ga, In, Y, La, Gd, Lu, Yb, Ce, Pr, Sm, Eu, Dy, Ho, Er, Tm, Na, K, Rb, and Cs in the mixture in a quantity sufficient to act as a flux; and (4) firing the mixture in a reducing atmosphere at a temperature and for a time sufficient to convert the mixture to a rare earth-activated terbium-containing garnet scintillator.
In another aspect of the present invention, a solution of amounts of oxygen-containing compounds of at least one first metal selected from the group consisting of terbium and lutetium; at least one other rare earth metal selected from the group consisting of Ce, Pr, Nd, Sm, Eu, Dy, Ho, Er, and Tm; and oxygen-containing compounds of at least one second metal selected from the group consisting of Al, Ga, and In is precipitated in a basic solution to obtain a mixture of hydroxides of the metals. The amounts of oxygen-containing compounds are selected such that the final composition of the scintillator is achieved. The mixture of precipitated hydroxides is calcined in an oxidizing atmosphere. The calcined material is further thoroughly mixed, and then fired in a reducing atmosphere at a temperature and for a time sufficient to convert the calcined mixture to rare earth-activated terbium containing garnet scintillator.
In still another aspect of the present invention, an X-ray detector is provided and comprises a scintillator comprising terbium-containing garnet activated with at least one rare-earth metal having a general formula of (G1-x-yAxREy)aDzO12, wherein G is at least one metal selected from the group consisting of terbium and lutetium; A is a member selected from the group consisting of Y, La, Gd, Lu, and Yb when G is Tb and selected from the group consisting of Y, La, Gd, Tb, and Yb when G is Lu; RE is at least one member selected from the group consisting of Ce, Pr, Nd, Sm, Eu, Dy, Ho, Er, and Tm; D is at least one member selected from the group consisting of Al, Ga, and In; a is in the range from about 2.8 to and including 3; x is in the range from 0 to about 0.5; y is in the range from about 0.0005 to about 0.2; and z is in the range from about 4 to and including 5. In one aspect of the present invention 4 less than z less than 5.
In still another aspect of the present invention, such an X-ray detector is incorporated in a CT system.
Other features and advantages of the present invention will be apparent from a perusal of the following detailed description of the invention and the accompanying drawings in which the same numerals refer to like elements.